As is understood by those in the industry, power inverters change direct current voltages into alternating current voltages. As is also understood, square wave inverters are more efficient than pulse width modulated inverters. However, in mid-frequency power inverters, the switching losses associated with the power transistors can become a dominant loss mechanism. Both "ON" and "OFF" switching transitions can be minimal when the effects of the parasitic circuit elements are used in an optimum manner.
In a square wave power inverter, a low impedance gate drive having switching edges on the order of 20 nanoseconds (nS) or less, is used to minimize switching losses when a heavy load is applied to the inverter. However, under no load or very light load conditions, the high speed at which the inverter switches causes ringing of the parasitic elements of the output power transformer. This ringing causes the output voltage to overshoot and peak charge the output capacitors, resulting in output voltages that may approach double the nominal value.
Therefore, an object of the invention is to provide a power inverter that can be run at full duty cycle over a widely varying load range.
Another object of the invention is to provide an efficient power inverter that can be used over a widely varying load range.
Another object of the invention is to permit a square wave power inverter to be employed in circuits where the output is not tightly regulated but must remain within a given range.
Another object of the invention is to reduce the tendency of a square wave inverter to "peak charge" and exceed its nominal output voltage when a light load is applied to the inverter output.
Another object of the invention is to slow the inverter output voltage transition when a light load is applied to the inverter output.
Another object of the invention is to provide a square wave power inverter that employs trapezoidal operation when a light load is connected to the output.
Another object of the invention is to provide a mechanism for controlling gate drive impedance into the switching transistors of a square wave power inverter.